The present invention relates to a method and apparatus for processing audio signals to produce two delayed, complementary pitch signals to produce chorus and other special effects.
When doubling audio signals or producing the effect known as "automatic double tracking", it is usually advantageous to have signals available which vary in terms of pitch as well as delay since this simulates the natural occurrence more effectively. For example, the effect known as "vibrato" is usually considered more desirable than the effect known as "tremolo" because, although both involve variations in the signal volume, "vibrato" also involves a variation in pitch.
Various signal processors have been proposed for changing the pitch of a recorded tape, for example, after or during the time that the tape is being replayed at a faster or slower speed so as to fit the recorded message into a particular length of time slot. Generally, when a given amount of information on a tape is compressed, certain portions of the original wave form (or the digital equivalent thereof) are discarded in order to attain the original (normal) pitch, whereas when such information is expanded, certain portions of the wave form are repeated in order to attain the original pitch. Devices for attaining such results are disclosed, for example, in the following U.S. Pat. Nos. 3,104,284; 3,816,664; 3,949,175 and 4,121,058. Such patents generally recognize the undesirable effects such as "glitches" which result from chopping of the waves and seek to deal with them. The present invention is not suitable for compressing or expanding a sound wave although it employs much of the digital type of equipment required for said prior patents. Thus, the technique of converting an analog signal to a digital signal, storing the digital signal in a delaying device, recovering the stored signal at uniform but different rate than the rate at which it was stored and converting the recovered digital signals back to an analog signal, is generally practiced in said patents.
U.S. Pat. No. 3,749,837 discloses a process of creating enhanced musical effects such as a chorus or Leslie effect with a similar type of apparatus by varying the time delay of the samples stored in the delay device. FIG. 2 of the patent employs two shift registers (delay devices), two clocks (one for each register) and a modulator fed directly to one clock and inverted and fed to the other clock so that the passage of the sampled values is speeded up through one delay register and slowed down through the other. The resultant outputs of the registers are combined to derive the final output. The frequency of the modulator is 3 to 12 Hz, whereas the frequencies of the clocks are 22 KHz. That clock associated with each shift register controls the sampling, input, shifting and output of the shift register.
Among the objects of the invention is to provide an improved apparatus for producing a chorus or Leslie effect. This and other objects of the present invention are achieved by providing apparatus comprising:
(A) sampling means for sampling said input signal at a sampling rate fc and for generating a respective digital signal representative of each successive said sample; PA1 (B) memory means for storing said digital signals as they are generated by said sampling means, said memory means being capable of storing a plurality of said digital signals; PA1 (C) signal removal means for: PA1 (D) said signal removal means including delay limiting means for causing the frequency of said first and second rates to switch whenever the delay of either of said analog signals with respect to said input signal exceeds predetermined limits; and PA1 (E) means for combining said first and second analog signals to form an audio frequency output signal having a desired tone effect.
(1) removing said digital signals from said memory at a first rate which switches between an increased frequency fi&gt;fc and a decreased frequency fd&lt;fc and generating a first analog signal as a function thereof, said first analog signal being substantially identical in shape to said input signal but being delayed with respect thereto and having a frequency which is different therefrom; PA2 (2) removing said digital signals from said memory at a second rate which switches between said increased frequency fi&gt;fc and said decreased frequency fd&lt;fc and generating a second analog signal as a function thereof, said second analog signal being substantially identical in shape to said input signal but being delayed with respect thereto and having a frequency which is different therefrom; said first and second rates always being different such that one of said rates is at said increased frequency fi whenever the other of said rates is at said decreased frequency fd;
and
As a result of this apparatus, and the method carried out thereby, the present invention insures that the rate at which information is read out of the memory is maintained within predetermined limits so that both analog signals are phase continuous and devoid of glitches. Accordingly, the present invention makes it possible to produce the desired chorus, tremolo and other effects without the drawbacks of the prior art devices.